Microporous layer assembly and method of making the same

ABSTRACT

One embodiment includes a process including coating a first microporous layer onto a first decal and curing the first microporous layer and the first decal.

TECHNICAL FIELD

The field to which the disclosure generally relates includes fuel celllayers including microporous layers and products including the same andmethods of making the same.

BACKGROUND

Hydrogen is a very attractive fuel because it is clean and can be usedto efficiently produce electricity in a fuel cell. The automotiveindustry expends significant resources in the development of hydrogenfuel cells as a source of power for vehicles. Such vehicles would bemore efficient and generate fewer emissions than today's vehiclesemploying internal combustion engines.

A hydrogen fuel cell is an electrochemical device that includes an anodeand a cathode with an electrolyte therebetween. The anode receiveshydrogen-rich gas or pure hydrogen and the cathode receives oxygen orair. The hydrogen gas is oxidized in the anode to generate protons andelectrons. The protons pass through the electrolyte to the cathode. Theprotons react with the oxygen and the electrons in the cathode togenerate water. The electrons from the anode cannot pass through theelectrolyte, and thus are directed through a load to perform work beforebeing sent to the cathode. The work may be used to operate a vehicle,for example.

Proton exchange membrane (PEM) fuel cells are popular for vehicleapplications. The PEM fuel cell generally includes asolid-polymer-electrolyte proton-conducting membrane, such as aperfluorosulfonic acid membrane. The anode and cathode typically includefinely divided catalytic particles, usually platinum (Pt), supported oncarbon particles and mixed with an ionomer. The catalytic mixture isdeposited on opposing sides of the membrane. The combination of theanode catalytic mixture, the cathode catalytic mixture, and the membranedefine a membrane electrode assembly (MEA). MEAs are relativelyexpensive to manufacture and require certain conditions for effectiveoperation. These conditions include proper water management andhumidification, and control of catalyst poisoning constituents, such ascarbon monoxide (CO).

Several fuel cells are typically combined in a fuel cell stack togenerate the desired power. The fuel cell stack includes a series offlow field or bipolar plates positioned between the MEAs in the stack.The bipolar plates include an anode side and a cathode side for adjacentfuel cells in the stack. The fuel cell stack receives an anode hydrogenreactant gas that flows into the anode side of the stack. Anode gas flowchannels are provided on the anode side of the bipolar plates that allowthe anode gas to flow to the anode side of the MEA. The fuel cell stackreceives a cathode reactant gas, typically a flow of air forced throughthe stack by a compressor. Cathode gas flow channels are provided on thecathode side of the bipolar plates that allow the cathode gas to flow tothe cathode side of the MEA. Not all of the oxygen is consumed by thestack and some of the air is output as a cathode exhaust gas that mayinclude liquid water as a stack by-product. The bipolar plates may alsoinclude flow channels for a cooling fluid. An automotive fuel cellstack, for example, may include about two hundred or more bipolarplates.

SUMMARY OF EXEMPLARY EMBODIMENTS OF THE INVENTION

One embodiment includes a process including coating a first microporouslayer onto a first decal and curing the first microporous layer and thefirst decal.

Other exemplary embodiments of the invention will become apparent fromthe detailed description provided hereinafter. It should be understoodthat the detailed description and specific examples, while disclosingexemplary embodiments of the invention, are intended for purposes ofillustration only and are not intended to limit the scope of theinvention.

BRIEF DESCRIPTION OF THE DRAWINGS

Exemplary embodiments of the present invention will become more fullyunderstood from the detailed description and the accompanying drawings,wherein:

FIG. 1 illustrates a process according to one embodiment.

FIG. 2 illustrates a process according to one embodiment.

FIG. 3 illustrates a process according to one embodiment.

FIG. 4 illustrates a process according to one embodiment.

FIG. 5 illustrates a process according to one embodiment.

FIG. 6 illustrates a product according to one embodiment.

FIG. 7 illustrates a product according to one embodiment.

FIG. 8 illustrates a product according to one embodiment.

FIG. 9 is a graph of potential versus current density.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

The following description of the embodiment(s) is merely exemplary innature and is in no way intended to limit the invention, itsapplication, or uses.

In one embodiment, a method is provided for fabricating an MEA 25 havingmicroporous layers. Referring to FIG. 1, a decal blank 26 is provided.In one embodiment, the decal blank 26 may be expandedpolytetrafluoroethylene (ePTFE), polytetrafluoroethylene (PTFE), orKapton polyimide film (poly(4,4′-oxydiphenylene-pyromellitimide))(available from Dupont). Then a first microporous layer 22 is coatedonto the decal 26. In one embodiment, the first microporous layer 22 maybe coated by using a microporous layer ink. In one embodiment, the firstmicroporous layer 22 may be coated by Meyer rod and then allowed to dry.Next the decal 26 with the first microporous layer 22 may be placed inan oven and cured at a first temperature. In one embodiment, the firsttemperature may be the glass transition temperature (T_(g)) of a polymerin the microporous layer ink. In one embodiment, the polymer in themicroporous layer ink may be a fluoropolymer. After a suitable curingtime, for example about 15 minutes to about 60 minutes, the decal 26with the first microporous layer 22 may be cooled to room temperature.

Referring to FIG. 2, a cathode electrode or cathode catalyst layer 18 isthen coated over the first microporous layer 22. The first microporouslayer 22 may adhere to the cathode catalyst layer 18. In one embodiment,the catalyst layer 18 may be coated using a catalyst ink. Referring toFIG. 3, a membrane 12 is then provided over the cathode catalyst layer18. In one embodiment, the membrane 12 may be a polymer electrolytemembrane. The decal 26 with the first microporous layer 22 and thecathode catalyst layer 18 is hot pressed to the membrane 12 such thatthe cathode catalyst layer 18 is in contact with a first face 14 of themembrane 12. As shown in FIG. 4, the decal 26 then may be peeled away ifdesired and the first microporous layer 22 and the cathode catalystlayer 18 remain attached to the membrane 12. Conditions of temperature,pressure, and time for hot pressing known in the art may be used. Forexample, the hot pressing conditions may include a pressing time of 4minutes at 295° F. and 250 psi.

The process just described may then be repeated with a second decalblank 27, a second microporous layer 24, and an anode electrode or anodecatalyst layer 20. As shown in FIG. 5, the second microporous layer 24and the anode catalyst layer 20 may be hot pressed to a second face 16of the membrane 12. Then the second decal 27 is peeled away. In anotherembodiment, the decal 26 with the first microporous layer 22 and thecathode electrode 18, and the decal 27 with the second microporous layer24 and the anode electrode 20, may be simultaneously hot pressed toopposite sides of the membrane 12.

As shown in FIGS. 6-7, a cross-sectional view of the resulting product10 shows, starting from the bottom, the first microporous layer 22, thecathode catalyst layer 18, the membrane 12, the anode catalyst layer 20,and the second microporous layer 24. In one embodiment, the thickness ofthe first and second microporous layers 22, 24 may be about 2 μm toabout 100 μm. In another embodiment, the thickness of the first andsecond microporous layers 22, 24 may be about 15 μm to about 40 μm.FIGS. 6-7 may be described as an MEA 25 with microporous layers 22, 24.Referring to FIG. 6, in one embodiment the membrane 12 may extend pastthe microporous layers 22, 24 and the catalyst layers 18, 20. Referringto FIG. 7, in one embodiment the membrane 12 and the microporous layers22, 24 may extend past the catalyst layers 18, 20. In variousembodiments, the membrane 12 and the microporous layers 22, 24 may ormay not have the same active area. In various embodiments, theelectrodes 18, 20 and the microporous layers 22, 24 may or may not havethe same active area.

Referring to FIG. 8, in one embodiment a first gas diffusion media layer52 may be positioned over the first microporous layer 22, and a secondgas diffusion media layer 54 may be positioned over the secondmicroporous layer 24. Referring still to FIG. 8, in one embodiment, theproduct 10 may be a fuel cell 28 including the MEA 25 with microporouslayers 22, 24 adhered to the catalyst layers 18, 20, respectively, asdescribed above. The fuel cell 28 may include at least one bipolar plate30. The bipolar plate 30 may have a first face 32 and a second face 34.The bipolar plate 14 may include two sheets 36 and 38. The two sheets 38and 38 may be machined or stamped. The two sheets 36 and 38 may bewelded together. A reactant gas flow field comprising flow channels 40and lands 42 is defined in the first face 32. Cooling fluid flowchannels 44 may be provided over the second face 34.

Referring still to FIG. 8, in one embodiment of the invention the fuelcell 28 may include two bipolar plates 30, a first fuel cell bipolarplate 46 and a second fuel cell bipolar plate 48. The bipolar plates 46,48 may include a variety of materials including, but not limited to, ametal, metal alloy, and/or electrically conductive composite. In oneembodiment, the bipolar plates 46, 48 may be stainless steel. In oneembodiment, a method includes positioning the first bipolar plate 46over the first gas diffusion media 52, and positioning the secondbipolar plate 48 over the second gas diffusion media 54.

A soft goods portion 50 is provided between the first bipolar plate 46and the second bipolar plate 48. The soft goods portion 50 may includethe polymer electrolyte membrane 12 comprising the first face 14 and thesecond face 16. The cathode electrode 18 may adhere to the first face 14of the polymer electrolyte membrane 12, and the first microporous layer22 may adhere to the cathode electrode 18. The first gas diffusion medialayer 52 may overlie the first microporous layer 22. The first gasdiffusion media layer 52 may be hydrophobic. The first bipolar plate 46may overlie the first gas diffusion media layer 52.

The anode electrode 20 may adhere to the second face 16 of the polymerelectrolyte membrane 12, and the second microporous layer 24 may adhereto the anode electrode 20. The second gas diffusion media layer 54 mayunderlie the second microporous layer 24. The second gas diffusion medialayer 54 may be hydrophobic. The second bipolar plate 48 may underliethe second gas diffusion media layer 54.

Referring again to the microporous layers 22, 24, in one embodiment themicroporous layer 22 or 24 may include a plurality of particles, forexample including graphitized carbon or carbon blacks and a binder. Inone embodiment the binder may include a hydrophobic agent or polymersuch as, but not limited to, polyvinylidene fluoride (PVDF),fluoroethylene propylene (FEP), polytetrafluoroethylene (PTFE), or otherorganic or inorganic hydrophobic materials. The particles and binder maybe included in a liquid phase which may be, for example, a mixture of anorganic solvent and water to provide dispersion. In various embodiments,the solvent may include at least one of 2-propanol, 1-propanol orethanol, etc. In one embodiment, the microporous layers 22, 24 may havea thickness ranging from about 2 to about 100 micrometers, may include60-90 weight percent particles and 10-40 weight percent binder. Themicroporous layers 22, 24 may have an average pore size ranging fromabout 50 nm to about 100 nm.

In another embodiment, a microporous layer ink was formed by mixing thefollowing in a jar mill at 120 rpm with 30 ml grinding media for about20 hours: 2.4 g acetylene black, 37 ml deionized H₂O, 32 ml isopropylalcohol, and 0.2 g (NH₄)₂CO₃. Then 2.21 g of a 36 wt % polyvinylidenefluoride (PVDF) solution in IPA was added to the microporous layer ink.The bottle was then shaken by hand. Then the mixture was coated onto anexpanded PTFE decal with a Meyer Rod, size 70. The decal was allowed toair dry and then was placed in an oven preheated to 150° C. and allowedto soak for 1 hour. After curing the polymer, the decal and microporouslayer combination was coated with an appropriate catalyst ink and theMEA was fabricated in the traditional decal transfer process. In oneembodiment, appropriate substitute materials for acetylene black mayinclude, but are not limited to, XC-72, Black pearls, and Ketjen Black.In another embodiment, appropriate substitute materials for PVDF mayinclude, but are not limited to, any fluorinated polymer that has amelting point below that of the decal material, for example fluorinatedethylenepropylene (FEP) and fluorinated acrylates. In anotherembodiment, appropriate substitute materials for the pH-adjustingmaterial, (NH₄)₂CO₃, may include, but are not limited to, a basiccomponent that does not poison ionomer, for example NH₄OH.

FIG. 9 shows a comparison of an embodiment of the invention with otherfuel cell arrangements. The polarization curve shown is run on a 50 cm²platform, and the testing conditions are as follows: 270 kpa absolute,100% inlet RH, 60° C. cell temperature, and 2/2 stoichiometry ofhydrogen and air. This condition is running fully saturated at theinlets and there is liquid water being condensed in the cell as water isbeing produced; thus this condition stresses the ability of themicroporous layer and diffusion media to remove water. The embodiment ofthe invention represented in FIG. 9 is a fuel cell 28 with the firstmicroporous layer 22 on the MEA 25, and is represented by the lines withsquare markers. The lines with triangle markers represent a fuel cellwith a commercially available freestanding microporous layer. The lineswith circle markers represent a fuel cell with the microporous layerformed over the diffusion media layer, which is known in the art. Thedata points corrected by the high frequency resistance (HFR), which aredesignated as the “IR Free” curves, show that there is no additionaltransport resistance for the microporous layer on the MEA compared withthe microporous layer on the diffusion media layer. As shown in FIG. 9,the fuel cell with the microporous layer on the MEA performs as well asthe fuel cell with the microporous layer on the diffusion media up to acurrent density of about 1.2 A/cm² and performs better than thecommercially available freestanding microporous layer at a currentdensity greater than 1.2 A/cm².

During fuel cell operation, many stresses may be placed upon the MEA 25.For example, the MEA may take up water, stretch, shrink, compress, feeltension, and so forth. In one embodiment, the microporous layers 22, 24may add durability to the MEA 25 or the fuel cell 28 when the catalystlayers 18, 20 are coated directly over the microporous layers 22, 24.The microporous layers 22, 24 may lend mechanical support to the MEA 25.In other embodiments, the microporous layers 22, 24 may slow down thedrying-out process of the MEA 25 and may protect the MEA 25 fromcatastrophic failure. The microporous layers 22, 24 may protect themembrane 12 from being punctured by fibers from the diffusion medialayers 52, 54. In another embodiment, the catalyst layers 18, 20 coatedover the microporous layers 22, 24 may contribute to water managementwithin the fuel cell 28 by creating a tight interface without voids.This may aid in water removal from the fuel cell 28.

In various embodiments, the membrane 12 may comprise a variety ofdifferent types of membranes. In one embodiment, the membrane 12 may bea perfluorinated sulfonic acid polymer electrolyte in which the entiremembrane structure has ionic exchange characteristics. These membranesare commercially available, and a typical example of a commercialsulfonic perfluorocarbon proton conductive membrane is sold by E. I.DuPont D Nemours & Company under the trade designation NAFION. Othersuch membranes are available from Asahi Glass and Asahi ChemicalCompany. The use of other types of membranes 12, such as, but notlimited to, perfluorinated cation-exchange membranes, hydrocarbon basedcation-exchange membranes as well as anion-exchange membranes are alsowithin the scope of the invention.

In one embodiment, the first gas diffusion media layer 52 or the secondgas diffusion media layer 54 may include any electrically conductiveporous material. In various embodiments, the gas diffusion media layer52 or 54 may include non-woven carbon fiber paper or woven carbon clothwhich may be treated with a hydrophobic material, such as, but notlimited to, polymers of polyvinylidene fluoride (PVDF), fluoroethylenepropylene, or polytetrafluoroethylene (PTFE). The gas diffusion medialayer 52 or 54 may have an average pore size ranging from 5 to 40 μm.The gas diffusion media layer 52 or 54 may have a thickness ranging fromabout 100 to about 500 μm.

In one embodiment, the cathode electrode 18 and the anode electrode 20(cathode layer and anode layer) may be catalyst layers which may includecatalyst particles such as platinum, and an ion conductive material suchas a proton conducting ionomer, intermingled with the particles. Theproton conductive material may be an ionomer such as a perfluorinatedsulfonic acid polymer. The catalyst materials may include metals such asplatinum, palladium, and mixtures of metals such as platinum andmolybdenum, platinum and cobalt, platinum and ruthenium, platinum andnickel, platinum and tin, other platinum transition-metal alloys, andother fuel cell electrocatalysts known in the art. The catalystmaterials may be finely divided if desired. The catalyst materials maybe unsupported or supported on a variety of materials such as, but notlimited to, finely divided carbon particles.

When the terms “over”, “overlying”, “overlies”, or “under”,“underlying”, “underlies” are used with respect to the relative positionof a first component or layer with respect to a second component orlayer, such shall mean that the first component or layer is in directcontact with the second component or layer, or that additional layers orcomponents are interposed between the first component or layer and thesecond component or layer.

The above description of embodiments of the invention is merelyexemplary in nature and, thus, variations thereof are not to be regardedas a departure from the spirit and scope of the invention.

1. A process comprising: coating a first microporous layer onto a firstdecal; and curing the first microporous layer and the first decal.
 2. Aprocess as set forth in claim 1 further comprising: coating a firstcatalyst layer over the first microporous layer so that the firstcatalyst is adhered to the first microporous layer.
 3. A process as setforth in claim 2 further comprising: providing a membrane comprising afirst face and a second face; and hot pressing the first microporouslayer and the first catalyst layer to the first face of the membrane. 4.A process as set forth in claim 3 further comprising: coating a secondmicroporous layer onto a second decal; curing the second microporouslayer and the second decal; coating a second catalyst layer over thesecond microporous layer; and hot pressing the second microporous layerand the anode catalyst layer to the second face of the membrane.
 5. Aprocess as set forth in claim 2 further comprising: coating a secondmicroporous layer onto a second decal; curing the second microporouslayer and the second decal; coating a second catalyst layer over thesecond microporous layer; providing a membrane comprising a first faceand a second face; and simultaneously hot pressing the first microporouslayer and the first catalyst layer to the first face of the membrane,and the second microporous layer and the anode catalyst layer to thesecond face of the membrane.
 6. A process as set forth in claim 5wherein the first decal and the second decal comprise one of expandedpolytetrafluoroethylene (ePTFE), polytetrafluoroethylene (PTFE), orpoly(4,4′-oxydiphenylene-pyromellitimide).
 7. A process as set forth inclaim 1 wherein the first microporous layer and the first decal arecured at the glass transition temperature of a polymer in the firstmicroporous layer.
 8. A process as set forth in claim 5 wherein thesecond microporous layer and the second decal are cured at the glasstransition temperature of a polymer in the second microporous layer. 9.A process as set forth in claim 3 wherein the membrane comprises apolymer electrolyte membrane.
 10. A process as set forth in claim 5wherein the first and second catalyst layers comprise catalyst particlesand an ion conductive material.
 11. A process as set forth in claim 5wherein the first and second microporous layers comprise carbon black.12. A process as set forth in claim 5 further comprising: removing thefirst decal and providing a first gas diffusion media layer over thefirst microporous layer; providing a first bipolar plate over the firstgas diffusion media layer, wherein the first bipolar plate includes aplurality of lands and channels; removing the second decal and providinga second gas diffusion media layer over the second microporous layer;and providing a second bipolar plate over the second gas diffusion medialayer, wherein the second bipolar plate includes a plurality of landsand channels.
 13. A process as set forth in claim 1 wherein the firstmicroporous layer comprises at least one of graphitized carbon or carbonblack and a hydrophobic agent.
 14. A process as set forth in claim 13wherein the hydrophobic agent comprises a fluorocarbon.
 15. A productcomprising: a decal; and a microporous layer overlying the decal.
 16. Aproduct as set forth in claim 15 further comprising a catalyst layeradhered to the microporous layer.
 17. A product as set forth in claim 16further comprising a membrane overlying the catalyst layer.
 18. Aproduct as set forth in claim 16 wherein the catalyst layer comprisescatalyst particles and an ion conductive material.
 19. A product as setforth in claim 15 wherein the microporous layer comprises at least oneof carbon black or graphitized carbon.
 20. A product as set forth inclaim 19 wherein the microporous layer further comprises at least one ofpolyvinylidene fluoride, fluoroethylene propylene,polytetrafluoroethylene, organic hydrophobic material, or inorganichydrophobic material.
 21. A product as set forth in claim 15 wherein themicroporous layer has an average pore size ranging from about 50 toabout
 100. 22. A product comprising a microporous layer adhered to acatalyst layer.
 23. A product as set forth in claim 22 furthercomprising a membrane overlying the catalyst layer.
 24. A product as setforth in claim 23 further comprising a gas diffusion media layeroverlying the microporous layer.
 25. A product as set forth in claim 24further comprising a bipolar plate overlying the gas diffusion medialayer, wherein the bipolar plate includes a plurality of lands andchannels.